Leishmania is a parasitic protozoan which spends part of its life cycle as an obligate intracellular parasite within the macrophage of its mammalian host. Mammalian host infection occurs during a bloodmeal of a phlebotomine sandfly carrying the flagellated promastigote form of organism. The promastigote enters the macrophage where it undergoes differentiation to the nonflagellated amastigote form. A number of complementary methods will be used to study the molecular events involved in the transformation from promastigote to amastigote and to determine how this transition permits maintenance of a successful parasitic interaction between the parasite and the macrophage. Recombinant DNA libraries generated from mRNAs isolated from promastigotes and amastigotes will be used to identify sequences which are present exclusively in one or the other form. cDNA molecules containing these sequences will be used to analyze regulation of gene expression during the promastigote to amastigote differentiation. The sequences containing a portion of the coding information for those polypeptides expressed exclusively in each form will be inserted into the plasmid expression vector PMR100 to generate fusion proteins containing Beta-galactosidase. These fusion proteins will be used to make antibodies specific for parasite antigens. The antibodies will then be used to follow differentiation of the organism during infection, to determine the mechanism by which survival in the macrophage is effected, and to define alterations which occur in the surface of the host cell.